1. Field of the Invention
The present invention relates to a detecting technique for measuring recording position error from a test pattern recording result recorded with a recording head (for example, an inkjet head) having a plurality of recording elements, and an image forming technique to which the detecting technique is applied.
2. Description of the Related Art
Methods of recording an image on a recording medium such as a recording paper include an inkjet drawing system in which ink drops are ejected from a recording head in accordance with an image signal and the ink drops are deposited on the recording medium. An example of an image drawing apparatus using such an inkjet drawing system is a full-line head image rendering apparatus in which an ejecting unit (a plurality of nozzles) that ejects ink drops is linearly disposed to correspond to an entire area of one side of a recording medium, and the recording medium is conveyed in a direction perpendicular to the ejecting unit in order to enable an image to be recorded on an entire area of the recording medium. Since a full-line head image rendering apparatus is capable of rendering an image on an entire area of a recording medium by conveying the recording medium without moving an ejecting unit, the full-line head image rendering apparatus is suitable for increasing recording speed.
However, with a full-line head image rendering apparatus, a deviation of an actual dot position that is recorded on a recording medium from an ideal dot position due to various reasons such as production variation, deterioration with age, or the like of recording elements (nozzles) constituting an ejecting unit may cause a recording position error (deposited position error). As a result, a problem that streaky artifacts occur in an image recorded on the recording medium arises. In addition to artifacts due to such a recording position error, there are phenomena in which streaky artifacts occur in a recorded image on the recording medium due to failures in a recording element such as an abnormality in which droplets are not ejected (non-ejection), an abnormality in volume of ejected droplets, and an abnormality in shape of ejected droplets (splash). Such recording elements which cause a decline in recording quality are collectively referred to as “defective ejection nozzles” or “defective recording elements”.
A length of a full-line recording head is equivalent to a width of a recording paper. For example, when recording resolution is 1200 DPI, an apparatus that can deal with a recording paper having a paper width similar to that of half Kiku size (636 mm by 469 mm) has approximately 30,000 nozzles/ink as the recoding elements. In such a large number of recording elements, defective ejection nozzles may occur at various timings. More specifically, a nozzle may become defective at the time of manufacture of a recording head, a nozzle may become defective due to deterioration with age, a nozzle may become defective during maintenance (in case of maintenance-induced, the nozzle is often restored to a normal nozzle by a next maintenance), and a recording element may become a defective ejection nozzle midway through continuous printing.
A technique is known in which, when a defective ejection nozzle occurs, usage of the defective ejection nozzle is suspended (ejection suspension) and then other surrounding nozzles (nozzles capable of normal ejection) are used in order to correct an image. Applying the correction technique, it is important that a defective ejection nozzle is accurately identified.
As techniques for identifying a defective ejection nozzle, Japanese Patent Application Publication No. 2004-009474, Japanese Patent Application Publication No. 2006-069027, and Japanese Patent Application Publication No. 2007-054970 describe methods of identifying a defective ejection nozzle by printing a predetermined test pattern aimed at detection of a defective ejection nozzle, reading a printing result with an image reading apparatus, and analyzing obtained read image data.
Japanese Patent Application Publication No. 2004-009474 discloses technology using a so-called 1-on N-off detection test pattern. A reading apparatus (scanner) has a resolution equal to or greater than the printing resolution and detects non-ejecting nozzles by binarizing the read result.
Furthermore, Japanese Patent Application Publication No. 2006-069027 discloses technology which detects a defective nozzle position on the basis of an average value of read results of one row which is under consideration in a test pattern, and an average value of read results of m rows of the left and right from the row under consideration. Here, it is desirable that the reading resolution of the image reading unit should be n times the resolution of the line head (where n is a natural number not less than 2).
However, Japanese Patent Application Publication No. 2004-009474 and Japanese Patent Application Publication No. 2006-069027 do not disclose detection technology which addresses the issue of using a reading apparatus having a lower resolution than the print resolution of the line head.
In respect of this issue, Japanese Patent Application Publication No. 2007-054970 discloses technology for detecting defective nozzles by using a scanner which reads at a lower resolution than the resolution of a recording head and applying interpolation processing to the read data.
However, in the technology disclosed in Japanese Patent Application Publication No. 2007-054970, under conditions where the line width formed by dots on a test pattern does not satisfy the sampling theorem, the line position retains a certain amount of error (estimated error in the line profile formed by the dots), and thus there is a possibility that accuracy of the detection cannot be achieved to the high point.